Modeling the Syn Disposition of Nitrogen Donors at the Active Sites of Carboxylate-Bridged Diiron Enzymes. Enforcing Dinuclearity and Kinetic Stability with a 1,2-Diethynylbenzene-Based Ligand

The syn coordination of histidine residues at the active sites of several carboxylate-rich non-heme diiron enzymes
has been difficult to reproduce with small molecule model compounds. In this study, ligands derived from 1,8-naphthyridine, phthalazine, and 1,2-diethynylbenzene were employed to mimic this geometric feature. The
preassembled diiron(II) complex [Fe2(μ-O2CArTol)2(O2CArTol)2(THF)2] (1), where ArTolCO2- is the sterically hindered
carboxylate 2,6-di(p-tolyl)benzoate, served as a convenient starting material for the preparation of iron(II) complexes,
all of which were crystallographically characterized. Use of the ligand 2,7-dimethyl-1,8-naphthyridine (Me2-napy)
afforded the mononuclear complex [Fe(O2CArTol)2(Me2-napy)] (2), whereas dinuclear [Fe2(μ-DMP)(μ-O2CArTol)2(O2CArTol)2(THF)] (3) resulted when 1,4-dimethylphthalazine (DMP) was employed. The dinuclear core of compound
3 is kinetically labile, as evidenced by the formation of [Fe(O2CArTol)2(vpy)2] (4) upon addition of 2-vinylpyridine
(vpy). The diiron analogue of 4, [Fe2(μ-O2CArTol)2(O2CArTol)2(vpy)2] (5), was prepared directly from 1. When the
sterically more demanding ligand 2,6-di(4-tert-butylphenyl)benzoate (Ar4-tBuPhCO2-) was used, mononuclear
[Fe(O2CAr4-tBuPh)2(THF)2] (6) and [Fe(O2CAr4-tBuPh)2(DMP)2] (7) formed. The difficulty in stabilizing a dinuclear core
with these simple (N)2-donor ligands was circumvented by preparing a family of 1,2-diethynylbenzene-based ligands,
from which were readily assembled the complexes [Fe2(Et2BCQEBEt)(μ-O2CArTol)3](OTf) (15) and [Cu2(Et2BCQEBEt)(μ-I)2] (16), where Et2BCQEBEt is 1,2-bis(3-ethynyl-8-carboxylatequinoline)benzene ethyl ester. The Et2BCQEBEt
framework provides both structural flexibility and the desired syn nitrogen donor geometry, thus serving as a good
first-generation ligand in this class.